The present invention relates to a power transmission system for a four-wheel drive motor vehicle, and more particularly to a system having a fluid-operated multiple-disk friction clutch for distributing the power of an engine to front and rear wheels of the vehicle.
Japanese Patent Publication 61-28531 discloses a system in which a fluid-operated multiple-disk friction clutch is provided as a transfer device for distributing the power of the engine to the front wheels and rear wheels. The clutch is disposed in an extension case of an automatic transmission.
The clutch comprises a clutch drum and a clutch hub, each having a plurality of disks, and a piston oil chamber formed between the clutch drum and a piston. The piston is slidably mounted on the hub with splines for pushing the disks of the clutch drum to the disks of the hub. When the clutch hub rotates, the piston rotates together with the hub.
When the clutch rotates at a high speed, the centrifugal force of the oil in the piston oil chamber exerts on the piston. When disengaging the clutch, the pressure of oil is reduced. However, the centrifugal force acts to push the piston to the disks of the clutch drum, so that the clutch cannot be completely disengaged. Further, the centrifugal force causes various disadvantages in accurate control of the pressure of oil.
In order to provide a sufficient transmitting torque for the clutch, the diameter of the piston must be enlarged. However, since the piston is mounted on the clutch drum, an increase of the diameter of the piston entails an increase of the diameter of the clutch drum. If the clutch drum becomes large, the resistance of lubricating oil in the clutch during operation becomes large, which causes a reduction of power transmission efficiency.
The prior art has a further disadvantage as described below.
The multiple-disk friction clutch uses the oil in the automatic transmission as actuating oil for actuating and lubricating the clutch, since the oil has a good friction characteristic and a low oil viscosity. On the other hand, a differential provided between the right and left driving wheels comprises hypoid gears. In such a differential, there is an extreme-pressure lubricant which contains additives to improve the ability to adhere to the surfaces of metals under high bearing pressures. Such a lubricant is liable to cause the multiple disks of the clutch to stick and to slip, which causes an unpleasant vibration of the vehicle body. Further, the friction characteristic of the lubricant largely changes with temperature, which adversely affects durability and reliability of the clutch.
Japanese Patent Application Laid-Open 61-155027 discloses a system in which a pair of transfer clutches for the front wheels and for the rear wheels are provided in a transfer device provided for distributing the torque to the front wheels and the rear wheels.
Japanese Patent Application Laid-Open 61-146637 also discloses a system in which a transfer clutch is provided between a final reduction gear of a differential and a transfer device. A piston of the transfer clutch is mounted in the final reduction gear. Accordingly, in such systems, the centrifugal force is similarly exerted on the piston when the power transmission system rotates.